1. Field of the Invention
The present invention relates to illumination optical systems, for example illumination optical systems that are used to form alignment system illumination beams in lithography.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., photo resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers half of the width of the substrate, for example, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
It has become very important with the decrease in feature size to accurately align substrates before each exposure. An alignment illumination beam of radiation can be formed from multiple wavelengths of light that are combined together. For example, the light can be from four different optical fibers coupled to four light sources each producing a different wavelength of light. The four light beams are combined using an optical multiplexer. However, this combined light may not have a desired optical symmetry in order to produce the best alignment illumination beam for alignment operations. In particular, it is desired to have alignment illumination beam having 180 degree phase and amplitude symmetry to a very high tolerance. This can be difficult to achieve with the multiplexer because the combined beam formed by the multiplexer typically includes excessive high spatial frequency amplitude and phase variations and ripples, among other problems. These types of defects in an alignment illumination beam can limit 0-order extinction ratio, which can affect the ability to detect weak or shallow alignment marks on a substrate with the alignment illumination beam.
What is needed is a system and method that produces an alignment system illumination beam having 180 degree symmetry.